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The 66-year-old Florida woman was no stranger to pain, having long suffered a chronic nerve disorder called trigeminal neuralgia that causes sharp twinges on the right side of her face.

But this was even worse: excruciating jolts of pain that she described as "electrical," along the roof of her mouth. Oddly, she told doctors at the Mayo Clinic in Jacksonville, Fla., the attacks came whenever she ate tomatoes or other acidic foods.

Turns out it was electrical. A recent root-canal procedure had created in effect a crude battery in her mouth. The dental work had repositioned a mercury-amalgam filling closer to an adjacent tooth with a gold-alloy crown.

Acidic saliva caused a tiny current to pass between the two metals -- enough to trigger severe pain along her exquisitely sensitive facial nerve.

Adjacent dental fillings made of dissimilar metals, in contact with saliva, can form a galvanic "battery" generating electrical currents with potentials of less than a volt, said neurologist William Cheshire Jr., who reported the case in a recent letter to the New England Journal of Medicine. Such currents usually cause no symptoms, he said, though some patients report "a metallic or battery-like taste."

The upshot for his patient: Once her mercury amalgam was replaced with a porcelain restoration, the pain-galvanizing "battery" went dead, and she could safely return to eating tomatoes.

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